The flat display technologies have developed rapidly over the last decade, with significant improvements in screen size and display quality. The flat panels have achieved performance comparable to conventional CRT displays in various aspects and are gradually replacing the cumbersome CRT displays. Currently, liquid crystal displays (LCDs) and organic light emitting displays (OLEDs) are the mainstream flat panel displays. On the other hand, flexible displays with their bendable property are widely applied in areas requiring a curved display, such as smart cards, electronic papers, smart tags and so on, covering almost all the applications where the conventional display devices are applicable, and gradually becoming a fashion in the field of display technologies.
In a flexible display, as shown in FIG. 1, one important procedure during the fabrication of flexible display devices is to bond an Integrated Circuit (IC, e.g., IC-chip on glass (COG), which is a chip bonded directly on a carrier glass substrate and applicable to LCD and OLED of consumer electronics such as mobile phones, PDAs and other portable devices) or a flexible printed circuit (FPC) to a flexible substrate by using an anisotropic conductive film (ACF). Currently, a flexible substrate normally uses a plastic substrate as the base substrate; because the plastic substrate is generally very thin (with a thickness of 25 to 125 μm) and soft, it can be easily deformed under a large bonding pressure for bonding the IC or FPC to the flexible substrate, thereby affecting the bonding precision and the contact effect of the electronic devices, which may further lead to process issues such as misalignment and poor contact.
Nowadays two solutions are generally used to solve the above problem.
According to the first solution, the plastic substrate of the flexible substrate is first placed on a rigid carrier such as a glass substrate to achieve higher rigidity, which is convenient for bonding; the plastic substrate is then removed from the rigid carrier after the completion of the bonding process. Although the solution can ensure the rigidity of the plastic substrate, higher process stability and clean environment are required upon bonding the IC or PPC onto the plastic substrate, otherwise bonding bubbles may be easily formed at the bonding surface, which will in turn affect the subsequent processes.
The other solution replaces the flexible substrate with a thicker plastic substrate. However, the plastic substrates suitable for flexible substrates of display devices are expensive, remarkably increasing the costs of the flexible displays. Meanwhile, the thicker plastic substrates have the following disadvantages. First, there is big difference between the coefficients of thermal expansion (CTE) of the plastic and glass substrates. Therefore, in the heating process following the bonding process, the different coefficients of thermal expansion between the thicker plastic substrate and the glass substrate and the inherent heat-shrink property of the plastic substrate will cause the glass substrate to warp under the pulling by the plastic substrate, when the heating is finished. Second, a thicker plastic substrates has larger rigidity and can not entirely bond to a glass substrate, which will cause bonding bubbles at the bonding surface. The bubbles will aggregate and expand when heated in the vacuum, affecting the flatness between the two substrates. It therefore shows that the solution of application of thicker plastic substrates for flexible substrates needs to be improved as well.
With the increase in the production volume of flat panels, the competitions in the industry are getting stronger as well. Therefore, in order to increase the competitive power of the product, an urgent technical problem to be solved is to ensure an excellent bonding effect between the electronic device(s) and the flexible substrate such that the product performance can be improved, while reducing the production costs of the flexible displays.